The invention relates to a method and an apparatus for implementing network planning in a radio system in a desired area, where at least a two-dimensional vector map comprising a description of the buildings and surfaces reflecting radio waves in the area is used for determining the coverage area of a transmitter of the system.
When a radio system is being constructed, a desired coverage area is to be achieved with the lowest possible cost. When considering the locations of the base stations in the system the required traffic capacity and the achieved coverage area are taken into account. The aim is to locate the base stations so that an extensive coverage area is achieved and that the base station is advantageously located as far as the radio wave propagation is concerned.
Different methods and instruments have been created for radio network planning. Digital maps providing modeled information on the terrain and buildings in the desired area are generally used as instruments in radio network planning. By using a vector map a computer can be employed to calculate coverage areas and parameters associated with network operation for different base station locations.
Prior art solutions require a lot of calculation capacity and time, because each transceiver link is calculated in real time. Particularly when densely populated areas are modeled using ray tracing the calculation time is extensive, since the densely populated areas comprise a lot of surfaces and corners reflecting radio waves, from where the radio waves scatter. Thus, several possible signal paths are created that have to be taken into account in the calculation.
It is thus important to determine signal fading in different areas when determining the coverage area of the transmitter. When simulating the behaviour of the system signal fading has to be modeled as realistically as possible. Particularly in an urban environment fading is a quantity changing in time depending on the geometry of the buildings surrounding the base station and the terminal, the walls and other scattering surfaces.
Conventionally fading has been divided into two different types, fast and slow fading, but this is a very rough division. In reality, multipath fading of a signal is caused by phase differences, and when a terminal moves the change in phase differences causes reciprocating variation to the signal strength which can typically be seen at the correlation distances from half a wave length to hundreds of wave lengths depending on the environment.
In prior art solutions, in moving radio system simulators, such as the link-level simulators implemented for example by COSSAP, fadings are simulated by adding attenuation and fading to the transmitted signal. Fast fading is typically generated by simulating a stationary random process according to Rayleigh or Rice distributions. Fading is thereafter averaged using an appropriate filter.
However, prior art methods comprise several problems and deficiencies. In typical simulators, for instance in the link-level simulators implemented by COSSAP, the reception algorithms are realistic, but the problem is how realistic the channel model is. A realistic channel model can be achieved using a ray tracing, or beam, search channel model. However, this is obstructed by the computational complexity required by a ray tracing method that prevents the implementation of the method in practice.
It is an object of the invention to provide a method and an apparatus implementing the method so as to solve the above problems. This is achieved with the method of the invention for implementing network planning in a radio system in a desired area, where at least a two-dimensional vector map comprising a description of the buildings and surfaces reflecting radio waves in the area is used for determining the coverage area of a transmitter of the system, and where the signal strength of the transmitter is determined at different points in the desired area. The method of the invention comprise the steps of selecting a subset among the points within the area, determining path attenuation between all subset points using a ray tracing method, storing the calculated values in a memory, and, when performing network planning in the desired area, locating the transmitter and a receiver at the desired points in the area, and by means of the values stored in the memory searching for a given number of the most significant signal paths among the possible signal propagation paths between the transmitter and the receiver, and determining the signal strength of the transmitter on the signal paths determined at a location point of the receiver using the ray tracing method.
The invention further relates to an apparatus for implementing network planning in a radio system in a desired area comprising means for maintaining at least a two-dimensional vector map describing the desired area and comprising a description of the buildings and surfaces reflecting radio waves in the area, and means for determining the signal strength of a transmitter at different points in the desired area. The apparatus of the invention comprises means for maintaining information about a subset of the points in the area, the means for determining path attenuation between all subset points using a ray tracing method, the means for storing the calculated values, and, when performing network planning, the means for determining a given number of the most significant signal paths among the possible signal propagation paths between the transmitter and the receiver located at the desired points in the area using the values stored in a memory and the means for determining the signal strength of the transmitter on the signal paths determined at the receiver location point using the ray tracing method.
The preferred embodiments of the invention are disclosed in the dependent claims.
Several advantages are achieved with the method and apparatus of the invention. In the solution of the invention a set of points is selected from the area described on the map, and path attenuation between said points is calculated beforehand and the paths and attenuation values are stored in the memory. When actually planning the path said pre-calculated values can be utilized and the amount of calculation is thus reduced and the path planning is accelerated.